This invention relates to a ceramic composition and a process for use thereof. More specifically, this invention relates to a ceramic composition which uses a connected branch copolymer as a binder; this ceramic composition allows production of a slurry of relatively low viscosity, while giving good green strength in the bonded ceramic material prior to firing.
It is known to form a ceramic article by making up an aqueous slurry comprising water, a ceramic material such as aluminum, clay, as dispersed in our new binders. The slurry can then be spray dried so that it emerges in the form of substantially spherical granules, or otherwise formed into a desired shape. The resultant spray dried powder can then be dry extruded or molded to give a desired shape. Alternatively, the slurry can be poured into a mold and allowed to settle, thereby producing a coherent mass of ceramic material together with an upper layer of separated water, which is then removed by decantation. The coherent article produced is known as greenware. Such greenware is normally subjected to further forming operations, such as drilling to produce the final desired shape of the article, before the greenware is oven-fired at a very high temperature to produce the final ceramic article.
Such a manufacturing process imposes stringent demands upon the binder used. The slurry formed in the first step of the process is required to have a high solids content but a relatively low viscosity in order to assist the spray-drying process. The spray-dried powder is desirably self-lubricating, so that no additional lubricant is need during extrusion or molding. The binder is desirably such that no mold release compounds are needed during molding. It is also desirable that the binder introduce "spring-back" into the greenware so that the greenware can readily be extracted from molds or tooling or complicated forms. The greenware produced should have a high green density, a high green strength, and not be unduly brittle, since brittle greenware is difficult to machine and tends to chip. Finally, the binder chosen should be one which gives rapid burn-off during the final oven firing, and is non-reactive with the wide range of ceramic materials used comercially.
No existing ceramic binder adequately meets all these exacting requirements. One known binder is polyvinyl alcohol. Polyvinyl alcohol gives greenware with a high green strength and is useable with most important ceramic materials. However, polyvinyl alcohol does not produce self-lubricating properties in the spray-dried powder so that, in order to maintain maximum green density, an additional lubricant is needed. In addition, polyvinyl alcohol-bound ceramic compositions give very little spring-back when pressed, which leads to difficulties in trying to extract parts from certain types of tooling. When dry and isostatic tooling are used, polyvinyl alcohol-bound ceramic compositions have a tendency to stick to the mold, so that a mold release compound is needed during pressing. Finally, greenware produced using a polyvinyl alcohol binder is hard and brittle and consequently difficult to machine since it tends to chip. Polyvinyl alcohol also has the minor disadvantage that it must be dissolved in water before use.
Another type of binder used commercially in the manufacture of ceramic articles is polyalkylene glycol polymers. Most prior art binders of this type are essentially linear polyalkylene glycol polymers, usually made by polymerization of ethylene and/or propylene oxide; the commercially preferred linear polyalkylene glycol polymer binder is Polyethylene Glycol Compound 20M manufactured by Union Carbide Corporation, the assignee of this application. Polyalkylene glycol binders, such as Polyethylene Glycol Compound 20M, have the advantages of giving self-lubricating properties to the spray-dried powder, thereby eliminating the need for an additional lubricant, and giving greenware with high green density and medium green strength. No mold release compounds are needed during molding and the binder produces adequate spring back. In addition, greenware formed using this type of binder is not brittle so that the greenware can be machined without chipping, although the only moderate green strength of the greenware can present certain problems. Finally, polyalkylene glycol binders burn off rapidly and cleanly during oven firing.
The green strength of greenware produced using polyalkylene glycol binders can be improved by adding polyvinyl alcohol as a co-binder, typically in an amount of 10-20% by weight of the polyalkylene glycol polymer. The addition of polyvinyl alcohol as a co-binder does increase the green strength of the products, but has the disadvantage of making the greenware somewhat more brittle.
The main disadvantages of the prior art polyalkylene glycol polymers are their effect upon the viscosity of the slurry. In order to produce good binding properties in the greenware, it is necessary to use polyalkylene glycol binders having a relatively high molecular weight; Polyethylene Glycol Compound 20M has a molecular weight of about 20,000. Unfortunately, as the molecular weight of the polyalkylene glycol polymers rises, so does the viscosity of the slurry solution. In practice, a compromise has to be made between the need to use a high molecular weight polymer in order to improve greenware properties, and the need to keep molecular weight low in order to avoid undue elevation of the viscosity of the slurry. Moreover, in many cases, this compromise requires lowering the solids content of the slurry in order to keep its viscosity within acceptable limits. Such lowering of the solids content of the slurry is undesirable in that it makes the slurry more difficult to dry during the spray-drying step.
It might be thought the viscosity problems encountered with linear polyalkylene glycol polymer binders could be overcome by using a star polyalkylene glycol polymer binder. Star polymers comprise a number of molecular chains radiating from a central "hub". Such polymers may be produced either by initiating polymerization of a monomer with a molecule having a functionality of at least three; the residue of this polyfunctional molecule forms the hub of the star polymer. Alternatively, linear polyalkylene glycol polymers having the molecular weight desired for the arms of the star polymer may be prepared, and then linked together using an appropriate polyfunctional molecule. At low molecular weights, the star polymers do indeed have viscosities lower than comparable linear polymers. However, as the length of the branches are increased to produce molecular weights within the range used in ceramic binders, chain entanglement occurs between branches, resulting in viscosities higher than those for corresponding linear polymers--see Kurmar, N. Ganesh, J. Polymer Science, Macromolecular Reviews, 15, 255 (1980). Accordingly, the use of star polymers does not solve the viscosity problems encountered when using linear polyalkylene glycol polymers as binders in ceramic compositions.
Accordingly, there is a need for a polymeric binder for ceramic compositions which does not produce excessively high viscosities in the slurry but adequately fulfills the other requirements for such binders discussed above, and this invention provides compositions and processes using such a binder.